The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. UMTS Terrestrial Radio Access Network (UTRAN) is the radio network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio network of a LTE system.
In E-UTRAN user equipments (UE) 150 are wirelessly connected to radio base stations 130a-c (usually referred to as evolved NodeB (eNB)) as illustrated in FIG. 1. In E-UTRAN the radio bases stations 130a-c are directly connected to the core network (CN) 100 via the S1 interface. The radio base stations or eNBs are also connected to each other via the X2 interface. The Operation and Support System (OSS) 120 is logically connected to all the radio bases stations 130a-c, as well as to the CN, via the OSS interface. In UTRAN however, the radio base stations (usually referred to as NodeB (NB)) are connected to the CN via a Radio Network Controller (RNC) which controls the NBs connected to it.
In E-UTRAN, the decision to handover from the current serving eNB to a target eNB is made within the serving eNB and is made on the basis of measurements on the downlink (DL). These measurements are performed by the UE that measures the DL signals it receives from the different eNBs in its area. The current version of the 3GPP LTE specification does not specify exactly what measurements to base the handover (HO) decision on. Previous versions of 3GPP standards, e.g. versions describing the Wideband Code Division Multiple Access (WCDMA) based radio interface and its evolutions (High Speed Data Packet Access, Enhanced Uplink), offer the possibility to perform measurements of both the DL Signal To Interference and Noise Ratio (SINR) and of the power of the received DL signal, and it is thus the operator's choice which of these two measurements to consider for the HO decision. Hence, a similar approach in the 3GPP LTE standard is likely to be proposed.
Henceforth T×P stands for the eNB transmission power of the pilot signal and R×P stands for the power of the pilot signal from one eNB as it is received at the UE. In E-UTRAN R×P is called Reference Signal Received Power (RSRP) and in UTRAN it is called Received Signal Code Power (RSCP).
Some problems occur when the HO decision is made based on the power of the received pilot signal, R×P (or on the DL SINR). One situation, that is very likely to be envisaged within 3GPP LTE, is that the eNB offering the highest received pilot signal power (R×P) is not necessarily the one that exhibits the highest DL path gain (highest DL path gain is equivalent to lowest DL path loss, but path gain will be used throughout the document to avoid confusion). Another situation is that the eNB offering the best DL path gain to a specific UE does not always correspond to the one offering the best uplink (UL) path gain. Both these situations are examples which may result in a UE that suffers from high loss rates on the UL after HO. Moreover, it is likely that the UE generates quite significant levels of UL other-cell interference, as the UL offered by an adjacent eNB might be much better than the UL offered by the serving eNB.
In the following, the situations described above are explained with mathematical expressions. If the HO decision is taken on the basis of the R×P, a UE is doing a handover to a new cell when the pilot signal power received from an adjacent eNB “B”, R×PB, is higher than the pilot signal power received from the serving eNB R×PA, multiplied by an HO margin, HOmargin. Hence, the HO decision mechanism may be described by the following:
Handover from serving cell “A” to adjacent cell “B”, ifR×PB≧R×PA*HOmargin  [1]
It should be noted that linear values are used in the above formula.
The received pilot signal power, R×P, is given by:R×P=T×P*gDL  [2]
where T×P is the eNB transmission power of the pilot and gDL is the average downlink path gain between the UE and the eNB.
When eNBs “A” and “B” transmit their pilot signals with the same transmission power, then [1] can be written as:gDLB≧gDLA*HOmargin  [3]
where gDLB is the average downlink path gain for the link between the UE and the eNB “B” and gDLA is the one between the UE and the eNB “A”. In such a case the UE is indeed attached to the eNB that exhibits the best downlink path gain.
Situation 1:
Highest R×P but not highest DL path gain (assuming gDL=gUL)
The transmission power of pilots is not always set to the same level for all of the eNBs in an operator's network. When imaging the case depicted in formula [1], the expression [2] above will result in:
                                          TxP            B                                TxP            A                          ≥                                            g              DL              A                                      g              DL              B                                *                      HO            margin                                              [        4        ]            
If the transmission power of the pilot of eNB “B” is higher than the transmission power of the pilot of eNB “A” (T×PB≧T×PA), then the following is true:gDLB≦gDLA*HOmargin  [5]
In such a case the UE is not attached to the eNB with the best DL path gain although attached to the eNB with the highest received pilot signal power. If then the average uplink path gain, gUL, is in accordance with the average downlink path gain gDL, then formula [5] applies for the average uplink path gain:gULB≦gULA*HOmargin  [6]
which means that the UE, after a HO to eNB “B”, may cause the problems mentioned above of high loss rate or high interference to the adjacent non-serving eNB “A”.
The same reasoning as the one just described, also applies for the case when the DL SINR is used as the criterion for the HO decision. The problem becomes more complex due to the addition of another variable, which is the interference. In case the interference levels at cells “A” and “B” are equal, then the problem is reduced to the one described by formulas [1]-[6].
Situation 2:
Best DL path gain not always best UL path gain (assuming T×PB=T×PA)
As mentioned previously, the same problems may arise when doing a HO from cell “A” to a cell “B”, when the average UL path gain for links to UEs in “B” is not equal to the DL one. An example is when the receiving and transmitting antennas are configured in different ways (two receiving antennas but only one transmitting) or when they have different sensitivities. If the transmission power of pilots is set to similar levels at the different eNBs that are serving cell “A” and “B”, the formula [3] applies. So the average DL path gain is better for cell “B” than for cell “A”. But if the average UL path gain of the eNB “B” is lower than the DL one, then it is possible that the situation described by of formula [6] and the same problems of high loss rate or high UL interference as described above occur.
A combination of the two situations above will give a third situation when both transmission power of pilots differ and the average UL and DL path gain differ. The outcome of such a situation can be both positive and negative in regards to the HO. If cell “B” has a higher transmission power of the pilot than “A” as well as an average UL path gain that is lower than the DL one, then the problems of loss rate and UL interference deteriorates. On the other hand, if the average UL path gain is higher than the DL one, then the problems are alleviated.